Me. Larkum et al., PROPAGATION OF ACTION-POTENTIALS IN THE DENDRITES OF NEURONS FROM RATSPINAL-CORD SLICE CULTURES, Journal of neurophysiology, 75(1), 1996, pp. 154-170
1. We examined the propagation of action potentials in the dendrites o
f ventrally located presumed motoneurons of organotypic rat spinal cor
d cultures. Simultaneous patch electrode recordings were made from the
dendrites and somata of individual cells. In other experiments we vis
ualized the membrane voltage over all the proximal dendrites simultane
ously using a voltage-sensitive dye and an array of photodiodes. Calci
um imaging was used to measure the dendritic rise in Ca2+ accompanying
the propagating action potentials. 2. Spontaneous and evoked action p
otentials were recorded using high-resistance patch electrodes with se
parations of 30-423 mu m between the somatic and dendritic electrodes.
3. Action potentials recorded in the dendrites varied considerably in
amplitude but were larger than would be expected if the dendrites wer
e to behave as passive cables (sometimes little or no decrement was se
en for distances of >100 mu m). Because the amplitude of the action po
tentials in different dendrites was not a simple function of distance
from the soma, we suggest that the conductance responsible for the boo
sting of the action potential amplitude varied in density from dendrit
e to dendrite and possibly along each dendrite. 4. The dendritic actio
n potentials were usually smaller and broader and arrived later at the
dendritic electrode than at the somatic electrode irrespective of whe
ther stimulation occurred at the dendrite or soma or as a result of sp
ontaneous synaptic activity. This is clear evidence that the action po
tential is initiated at or near the soma and spreads out into the dend
rites. The conduction velocity of the propagating action potential was
estimated to be 0.5 m/s. 6. The amplitude of the dendritic action pot
ential could also be initially reduced more than the somatic action po
tential using 1-10 mM QX-314 (an intracellular sodium channel blocker)
in the dendritic electrode as the drug diffused from the dendritic el
ectrode toward the soma. Furthermore, in some cases the action potenti
al elicited by current injection into the dendrite had two components.
The first component was blocked by QX-314 in the first few seconds of
the diffusion of the blocker. 7. In some cells, an after depolarizing
potential (ADP) was more prominent in the dendrite than in the soma.
This ADP could be reversibly blocked by 1 mM Ni2+ or by perfusion of a
nominally Ca2+-free solution over the soma and dendrites. This sugges
ts that the back-propagating action potential caused an influx of Ca2 predominantly in the dendrites. 8. With the use of a voltage-sensitiv
e dye (di-8-ANEPPS) and an array of photodiodes, the action potential
was tracked along all the proximal dendrites simultaneously. The resul
ts confirmed that the action potential propagated actively, in contras
t to similarly measured hyperpolarizing pulses that spread passively.
There were also indications that the action potential was not uniforml
y propagated in all the dendrites, suggesting the possibility that the
distribution of Na+ channels over the dendritic membrane is not unifo
rm. 9. Calcium imaging with the Ca2+ fluorescent indicator Flue-3 show
ed a larger percentage change in fluorescence in the dendrites than in
the soma. Both bursts and single action potentials elicited sharp ris
es in fluorescence in the proximal dendrites, suggesting that the back
-propagating action potential causes a concomitant rise in intracellul
ar calcium concentration. This might have important consequences for t
he modulation of metabolic processes that in turn might affect the mod
ulation of synaptic transmission by the postsynaptic neuron.10. In sum
mary, the results indicate that the dendrites of motoneurons have nonu
niformly distributed Na+ and Ca2+ conductances. These are activated by
the spreading action potentials that are generated at or near the som
a and propagate back into the dendrites. This has important consequenc
es for the processing of synaptic input by motoneurons as well as for
their intrinsic firing properties.